The long-range aim of this project is to determine the neural basis of adaptive mechanisms responsible for establishing and maintaining appropriate performance levels in the oculomotor system of the rhesus monkey. The mechanism currently under study relates to the gain of the vestibulo-ocular reflex. One series of experiments showed that changes in the gain of the vestibulo-ocular reflex are "frequency selective"; adaptation during passive sinusoidal oscillation at one temporal frequency produced large changes in gain at that frequency and smaller changes at adjacent frequencies. A second series of experiments quantified the serious effects of bilateral flocculectomy on the adaptive mechanism and on the optokinetic response. A third series of experiments showed that adaptation of the vestibulo-ocular reflex causes some changes in the twitch of eye velocity evoked by single electrical shocks applied to the vestibular nerve; the changes occured in parts of the twitch that can be attributed to multisynaptic vestibular pathways to extraocular motoneurons.